This invention relates to closed loop temperature control and, in particular, to an easily calibrated temperature sensing and control circuit for continuously controlling the temperature of a system in real time without hysteresis.
Temperature control circuits are legion and are encountered daily. The typical home heating system includes a temperature sensor and a switch for turning a heater on or off. The cycling of the heater is a kind of hysteresis that enables one to provide a relatively low cost control circuit. Specifically, the temperature is maintained within a range by turning the heater on until a maximum temperature is reached and then turning the heater off until the temperature drops to a predetermined minimum. The system oscillates between the two boundaries.
For many applications, even commercial processes, such a control is entirely adequate. For other applications, a more precise control is necessary. It is known in the art to provide proportional control; e.g. U.S. Pat. No. 4,495,405 (Foster). Although the Foster patent describes a circuit as providing proportional control, the output signal to a heater turns the heater on and off at a very low frequency, below one hertz, because the output signal tracks temperature variations in the system.
U.S. Pat. No. 4,162,379 (Sebens et al.) describes a heater control circuit that continuously monitors the power applied to a heating element. Variations in the resistance of the heater element, due to temperature, aging, or other causes, are compensated by the feedback network. Systematic errors are also compensated.
U.S. Pat. No. 5,793,022 (Klinck et al.) discloses an adaptive temperature controller in which the gain and the time constant of the controller are variables that are calculated during the control process. The Klinck et al. patent, in effect, addresses the problem of calibration by making certain parameters variables. It is preferred to provide a simpler control circuit that is easily calibrated.
Many of the precise temperature control circuits of the prior art are relatively complex, not amenable to integration on a single chip, which makes the circuits expensive. Similarly, many of the precise temperature control circuits of the prior art are adapted for a particular application, in part due to the thermal inertia of the systems for which they were designed.
In view of the foregoing, it is therefore an object of the invention to provide an adaptive temperature control circuit in which the output signal is pulse width modulated at a high frequency.
Another object of the invention is to provide an adaptive temperature control circuit that can be implemented as a single integrated circuit.
A further object of the invention is to provide an adaptive temperature control circuit that can interface directly with a microprocessor for setting temperature.
Another object of the invention is to provide an adaptive temperature control circuit that can easily be calibrated.
A further object of the invention is to provide an adaptive temperature control circuit for which the voltage versus temperature curve is easily adjusted over a wide range of slopes.
Another object of the invention is to provide an adaptive temperature control circuit that can be used with systems of widely different response times.